In some sites, the amount of collected heat in a solar heat boiler inevitably repeats sudden increase and decrease in accordance with the amount of solar radiation varying suddenly in a short time due to the sunlight blocked by cloud or the like.
On the other hand, solar heat boilers are often introduced into a region called the Sunbelt, that is, a region where annual Direct Normal Irradiance (DNI) is beyond 2,000 kWh/m2, in order to obtain as large the annular total amount of collected heat as possible.
It is usually sunny in the Sunbelt throughout the year, and the amount of solar radiation hardly changes suddenly due to the change in weather. Thus, the stable amount of collected heat over time prevents the aforementioned problem from coming to the surface.
In regions other than the Sunbelt, for example, in Japan, however, the amount of solar radiation frequently changes suddenly in a day due to the change in weather or the movement of clouds so that sudden increase or decrease in the amount of collected heat may appear repeatedly. It is therefore important to take measures against such a problem.
Concentrating solar power generation plants are roughly classified into stand-alone type electric power generation plants and integrated type electric power generation plants. In the stand-alone type electric power generation plants, heat may be secured mostly by solar heat and partially backed up with fossil fuel etc. On the other hand, in the integrated type electric power generation plants, heat is secured mostly by fossil fuel or nuclear fuel and partially backed up with solar heat.
In each type of the stand-alone type electric power generation plants and the integrated type electric power generation plants, heat from the sunlight is collected and used as a heating source, and both the types may use substantially common solar collectors.
Solar collectors used typically include trough type ones, Fresnel type ones, tower type ones, etc. Ina trough type solar collector, a heat transfer tube is disposed above an inner circumferential curved surface of a reflecting mirror extending like a trough so that the sunlight can be collected in the heat transfer tube by the mirror. Thus, water circulating in the heat transfer tube is heated to generate steam. In a Fresnel type solar collector, a several number of reflecting mirrors having flat surfaces or slightly curved surfaces are arranged side by side at angles differing bit by bit from one to another, and a several number of heat transfer tubes are disposed above the group of the reflecting so that the sunlight can be collected in the heat transfer tube by the group of the reflecting to generate steam. In a tower type solar collector, a heat transfer tube panel is disposed on a tower having a predetermined height and a large number of reflecting mirrors (heliostats) are disposed on the ground surface so that the sunlight can be collected in the heat transfer tube panel by the group of the reflecting mirrors (heliostats) to generate steam.
Among them, in the trough type and the Fresnel type, the focal length is so short that the concentration ratio of the sunlight (the heat density in a heat collecting portion) is low. On the other hand, the tower type has an advantage that the focal length is so long that the concentration ratio of the sunlight (the heat density in a heat collecting portion) is high.
High heat density in a heat collecting portion leads to increase in the amount of collected heat per unit heat transfer area, so that higher-temperature steam can be obtained. However, when the heat density is simply increased to make a phase change from a water state to superheated steam, there arises a problem that a high-temperature area is formed locally to thereby cause damage to transfer tubes or the like.
In a thermal power generation boiler or the like, the amount of fuel is managed properly to avoid such damage to any heat transfer tube. In the case of solar heat, however, the heat input amount fluctuates so largely that it is difficult to avoid thermal damage to the heat transfer tubes.
To solve such a problem in the tower type with high heat density, a solar heat boiler configured as shown in FIG. 17 and FIG. 18 has been proposed, for example, in Patent Literature 1, Patent Literature 2, etc.
FIG. 17 is a schematic configuration diagram of a solar heat boiler. FIG. 18 is an enlarged schematic configuration diagram of a heat collecting device for use in the solar heat boiler.
In FIGS. 17 and 18, the reference numeral 1 represents a heat collecting device; 2, an evaporator; 3, a superheater; 4, a steam-water separation device; 5, a tower; 6, a heliostat; 7, the sun; 8, a steam turbine; 9, an electric power generator; and 11, a water supply pump.
As shown in FIG. 18, the heat collecting device 1 is functionally divided into the evaporator 2 and the superheater 3, and the steam-water separation device 4 is placed between the evaporator 2 and the superheater 3. The heat collecting device 1 is placed on the tower 5 which is about 30 to 100 meters high. Light from the sun 7 is reflected by the heliostats 6 placed on the ground, and condensed on the heat collecting device 1 so as to heat the evaporator 2 and the superheater 3. Superheated steam generated in the heat collecting device 1 is sent to the steam turbine 8 so as to rotate the electric power generator 9. Electric power is generated in such a mechanism.
Further, FIG. 19 is a schematic configuration diagram of a solar heat electric power generation system described in U.S. Pat. No. 7,296,410 (Patent Literature 3). In FIG. 19, the reference numeral 200 represents a solar heat electric power generation system; 201, a fluid channel; 202, a valve; 203, a pump; 204, a trough device; 205, a heat collection tube; 206, a solar heat collector; 207, a tower; 208, a low-temperature heat storage tank; 209, an intermediate heat storage tank; 210, a high-temperature heat storage tank; 211, a high-output generation device; 212, a turbine; and 213, an electric power generator.
In the solar heat electric power generation system, a thermal fluid stored in the low-temperature heat storage tank 208 is supplied to the trough devices 204 by the pump 203, and heated by heat derived from the condensed light of the sun 106. The thermal fluid further heated in the tower 207 is then sent to the high-temperature heat storage tank 210. The thermal fluid sent to the high-temperature heat storage tank 210 is sent to the high-output generation device 211 by the pump 203. The thermal fluid whose temperature has decreased due to heat exchange is returned to the low-temperature heat storage tank 208.
On the other hand, configuration is made in such a manner that steam generated by the high-output generation device 211 is sent to the turbine 212 so that electric power is generated by the electric power generator 213.
Further, FIG. 20 is a schematic configuration diagram of a solar heat/light collection plant described in U.S. Pat. No. 8,087,245 (Patent Literature 4). In FIG. 20, the reference numeral 301 represents a trough type collector; 302, a tower with heliostats; 303, a low-temperature heat storage; 304. a high-temperature heat storage; 305, an auxiliary device using fossil fuel; 306, a turbine; 307, an electric power generator; 308, a condenser; and 309, a pump.
In the solar heat/light collection plant, water is sent to the trough type collector 301 by the pump 309 and heated by the heat of the sun so as to generate saturated steam. The generated saturated steam is sent to the tower with heliostats 302. The turbine 306 is driven by the superheated steam generated thus, so as to generate electric power in the electric power generator 307.
The steam is returned to water in the condenser 308, and the water is supplied again by the pump 309. Further, the configuration includes a line in which the saturated steam from the trough type collector 301 is not circulated in the tower with heliostats 302 but is passed through the auxiliary device 305 using fossil fuel so as to generate superheated steam.